1. Field of the Invention
The present invention relates to a device for positioning a semiconductor wafer which can be preferably used in, for example, a manufacturing device or an inspection device which is used in a process for manufacturing semiconductor devices, the semiconductor wafer having a cut portion (an orientation flat or a notch). More particularly, the present invention relates to a positioning device which can be preferably employed in an exposing device (a stepper, an aligner or the like) which must have an excellent positioning accuracy.
2. Related Background Art
Hitherto, exposing devices and inspection devices for use in a process for manufacturing semiconductor devices, and, more particularly, in a lithography process, employ an orientation flat (hereinafter called "OF") or notch so as to position the wafer with respect to the position of the device. In particular, the alignment accuracy of exposing devices must be improved since the semiconductor devices have been densely integrated and precisely manufactured. Therefore, a demand of accurately positioning and mounting the wafer on a device has arisen. A positioning device of the type described above is disposed in a passage of the exposing device through which the wafer is conveyed, the positioning device being arranged to act in accordance with a method in which the wafer is abutted against a reference member which is able to come in contact with the circumferential portion and the cut portion. Another method has also been employed in which the cut portion is optically detected by irradiating an optical beam to the periphery portion. In accordance with the former method, the wafer can be easily broken and the resist can be easily separated because the wafer is brought into contact with the reference member. Therefore, the manufacturing yield will be deteriorated since with the above-described broken or separated resist dust adheres, as foreign materials, to the surface of the wafer. Therefore, the latter method is widely used at present.
FIG. 16 is a schematic view which illustrates the structure of a conventional positioning device. FIG. 17 is a cross sectional view taken along line C--C of FIG. 16. As shown in FIGS. 16 and 17, an XY-stage 100 is supported on a base plate 106 via a guide member 107 in such a manner that the XY-stage 100 is able to move in X and Y directions. Furthermore, a .DELTA..theta. stage 101 is disposed on the XY-stage 100 in such a manner that it is able to slightly rotate around origin 0 of the rectangular coordinate system XY. In addition, a stepping motor 105 is disposed on the lower surface of the .DELTA..theta. stage 101 in such a manner that its motor shaft is made substantially to coincide with the origin O. Furthermore, a wafer chuck (turn table) 102 is secured to the motor shaft of the stepping motor 105 so that a .theta.-stage capable of infinitely rotating while holding the wafer W is constituted. A vacuum attraction groove 102a is formed in the surface layer of the turn table 102. The pressure at a space surrounded by the groove 102a and the reverse side of the wafer W is reduced so that the wafer W is drawn to the surface of the turn table 102. An external shape measuring sensor 103 includes a halogen lamp 108 and a lens 109, the external shape measuring sensor 103 detecting the position of the wafer edge by projecting the contour of the wafer W onto a line image sensor 111 by irradiating the peripheral portion of the wafer with irradiating light from the reverse side via a mirror 110. Reference numeral 104 represents a wafer conveying belt which is moved downwards after the wafer edge has been moved to a position corresponding to the central portion of the external shape measuring sensor 103 so as to send the wafer W to the turn table 102.
In the thus constituted device, distance .rho. from the rotational center of the turn table 102 to the wafer edge is detected by the external shape measuring sensor 103 in accordance with each of the rotational angles while rotating the turn table 102. In accordance with data about the result of the above-described detection, the direction of the OF is detected before the rotational directional positioning (an OF alignment) is performed within a range of the established accuracy (determined in accordance with the resolution of the stepping motor 105) of the rotational angle of the turn table 102. Furthermore, the .DELTA..theta. stage 101 is swung in accordance with the above-described data so that the direction of the OF is further precisely aligned. Then, the XY-stage 100 is operated so as to move the wafer center to the position (the origin O) at which the rotational center of the turn table 102 has been positioned before the correction is performed. The quantity of correction of the XY-stage 100 in the directions X and Y is calculated by measuring, at that time, the above-described distance .rho. for three or four points (P.sub.1, P.sub.2 and P.sub.3 of FIG. 16) on the circumference except for the OF. As a result, the wafer W can be accurately positioned with respect to the rectangular coordinate system XY while preventing undesirable two-dimensional deviation (rotation included).
However, there arises a problem in that the resist layer formed on the surface of the wafer can be easily separated in the periphery portion and the separated resist will adhere to the surface of the wafer, causing the manufacturing yield to be deteriorated. In order to prevent the separation of the resist, a portion, having a predetermined width (about 1 to 7 mm), of the periphery is selectively exposed to light by rotating the wafer while applying exposing light emitted from an exclusive exposing device to the periphery of the wafer. Specifically, the structure comprises an illuminating portion (for example, an optical fiber) disposed closely to the periphery portion and arranged to emit the exposing light. The structure further comprises a light receiving portion disposed to confront the irradiating portion via the periphery portion and arranged to receive an exposing light beam which has not been stopped by the periphery portion. The illuminating portion and the light receiving portion are integrally constituted in such a manner that they are able to move in the radial direction of the wafer. When the periphery is exposed to light, the illuminating portion, the light receiving portion and the wafer are relatively moved in the radial direction in accordance with the change in the detection signal transmitted from the light receiving portion so that the portion, having the predetermined width, of the periphery is always exposed to light. Furthermore, a structure has recently been disclosed in which a periphery exposing device of the type described above is included in the above-described positioning device.
However, a problem arises in the thus constituted conventional structure in that the weight and load of the XY-stage 100 becomes excessively large because the .DELTA..theta. stage is disposed on the XY-stage 100. As a result, the size of the XY-stage cannot be reduced, the operating speed will be lowered and the servo following performance or the like can be deteriorated. In a case where a heavy and large-size stage of the type described above is operated during the periphery exposure operation or the inspection operation, another problem arises in that the alignment accuracy and inspection accuracy can be deteriorated due to vibrations generated when the stage is moved.
When the wafer is held by the wafer chuck, it is impossible to accurately align the wafer center to the central portion of the wafer chuck. Therefore, when the wafer edge is detected by using the external shape measuring sensor, the wafer is eccentrically rotated. As a result, the light receiving surface of the line image sensor must have a sufficient length in the radial direction of the wafer. For example, in a case of a 6-inch wafer having an OF, the usual OF portion must have the light receiving portion the length of which is about 6 mm in the radial direction of the wafer. Furthermore, it is actually necessary for the length of the light receiving surface to be about 20 mm because the distance (quantity of the deviation) between the wafer center and the central portion of the chuck must be taken into consideration. Therefore, a problem arises in that accurate positioning cannot be performed since the resolution and the linearity of the conventional analog sensor are insufficient and the halogen lamp cannot emit uniform irradiating light beams. Furthermore, a problem arises in that the time taken to detect the subject is too long since the CCD has a sensor resolution of about several micrometers and there is an influence of the sweeping frequency at the time of the detection operation.
In order to improve the positioning accuracy in the above-described device, an accurate encoder and a stepping motor must be used to position the turn table after the rotation. Therefore, the overall cost of the device cannot be reduced and the weight of the same cannot also be reduced.
In order to expose the periphery of the wafer to light by using the above-described exclusive exposing device, servo operation means must be individually employed which integrally operates the illuminating portion and the light receiving portion for the purpose of always exposing the predetermined width of the periphery portion to light. Furthermore, the servo control condition becomes too strict because the weight of the illumination portion increases if an optical system for equalizing the light intensity distribution of the exposing light beam is further employed. The servo control condition also becomes too strict when another optical system for reducing the numerical aperture of the exposing light beam is used, the optical system for reducing the numerical aperture being provided for the purpose of preventing a so-called reduction of the thickness which is a problem taking place at the development process in that a portion of the resist is removed since the exposing light beam is introduced into a further inside portion (toward the central portion of the wafer) over a predetermined exposure width when the periphery is exposed to an exposing light beam having a large numerical aperture (N.A) emitted from, for example, an optical fiber. In addition, the servo controlling condition must be strictly determined for the OF portion in comparison to the conditions for the circumferential portion. As a result, another problem arises in that the exposure width cannot be equalized in the OF portion.